1. Field of the Invention
The present invention relates to the field of synthetic polymeric membrane materials formed from casting polyvinylidene difluoride (PVDF) polymer solutions and/or dispersions. Membranes formed in accordance with the present invention are all highly porous. Both internally isotropic membranes and highly asymmetric PVDF membranes are disclosed. Membranes of the invention may be hydrophobic or hydrophilic. The membranes of the invention are useful in a variety of microfiltration applications.
2. Background of the Technology
Significant interest and efforts have been invested into the manufacture of PVDF polymer membranes. The basic reason for the interest in PVDF membranes as filters is that PVDF is resistant to oxidizing environments including ozone which is used extensively in the sterilization of water. PVDF is also resistant to attack by most mineral and organic acids, aliphatic and aromatic hydrocarbons, alcohols, and halogenated solvents. It is soluble in certain aprotic solvents, such as dimethylformamide, N-methyl pyrrolidone, and hot acetone. Further, PVDF has good physical properties over temperatures ranging from -50 to 140.degree. C.
Grandine prepared what many consider the first practical PVDF microporous membrane, as described in U.S. Pat. No. 4,203,848. The membrane was prepared through a wet-thermal type phase inversion process. In the process, the PVDF was dissolved in acetone, at its boiling point of 55.degree. C. After casting, the membrane was quenched both thermally and in a water/acetone (20/80 by volume) quench bath. Acetone is a good solvent for PVDF at 55.degree. C. but a poor one at room temperature, so in effect Grandine used a combination of thermal and liquid quench.
Benzinger cast PVDF ultrafilters from formulations that preferably used triethyl phosphate as solvent and a variety of hydroxy compounds as nonsolvents. See U.S. Pat. No. 4,384,047. Josefiak disclosed PVDF as one of several "porous shaped bodies" that is cast utilizing a thermal quenching process. See U.S. Pat. No. 4,666,607. U.S. Pat. No. 4,774,132 to Joffee et al. discloses preparation of activated and modified PVDF structures. Similarly, Degen et al., in U.S. Pat. No. 5,282,971 disclosed PVDF membranes modified to contain quaternary ammonium groups covalently bound to the membrane. In U.S. Pat. No. 5,019,260 to Gsell et al., PVDF filtration media having low protein affinities were disclosed.
Costar Corporation, in published PCT Application No. WO 93/22034, discloses the preparation of PVDF membranes that are alleged to possess improved flow rates. The membranes, however, appear to possess an entirely isotropic structure containing a dense array of closely aligned and contiguous polymer particles. The pores in the membrane appear structurally similar to a sintered metal.
Sasaki et al. disclosed a PVDF formulation in U.S. Pat. No. 4,933,081 and GB 2,199,786A, published Jul. 20, 1988 (the "Fuji patent"). The PVDF formulation of Example 1 had a relatively high polymer concentration (20%) that was exposed to 60.degree. C. air with 30% relative humidity. In addition, the Fuji formulations included a high concentration of polyvinylpyrrolidone, used as a co-solvent/swelling agent. The surface pores in the Fuji PVDF membrane appear to be between about 0.45 .mu.m and 0.65 .mu.m, and the thicknesses of the membranes ranges from 100 .mu.m to 110 .mu.m.
Membranes of the prior art have been rendered hydrophilic by any of several methods. Each of these methods for imparting hydrophilicity to membranes has inherent problems or difficulties. For example, where a membrane is post-treated with a moiety to impart hydrophilicity there is a potential that the moiety will leach and contaminate the sample. One can attempt to minimize leaching through crosslinking certain moieties on the surface of the cast membrane. For example, Roesink et al. in U.S. Pat. No. 4,798,847 (now Re. No. 34,296) disclose crosslinking PVP throughout the structure of polysulfone membranes. However, while crosslinking hydrophilic moieties to membranes may minimize leaching, it can add additional steps and complexities to the fabrication process of a membrane. Further, depending on the conditions required for the crosslinking, membrane strength and/or rigidity can be compromised.
Another approach to imparting hydrophilicity to membranes involves the inclusion of a hydrophilic moiety within the casting suspension. For example, Kraus et al. in U.S. Pat. Nos. 4,964,990 and 4,900,449 disclose formation of hydrophilic microfiltration membranes from hydrophobic polymers through inclusion in the casting solution of a hydrophilic polymer, such as polyethylene glycol or polyvinylpyrrolidone. The membranes prepared in accordance with the Kraus patents are, however, limited to polyethersulfone formulations, and are isotropic. They are therefore not well suited to applications that require asymmetric PVDF membranes.
With respect to structure, the membranes of the Sasaki patent are all disclosed to possess two degrees of asymmetry. In cross-section, the membranes have a microporous face and a coarse pore face. The diameter of the pores first decreases along a line from the microporous face to the coarse pore face, reaching a minimum pore size between the microporous face and the coarse pore face of the membrane. Thereafter, the pore sizes increase along a line toward the coarse pore surface, but the increase, and hence the asymmetry of the membrane, is not as dramatic as in a classic Wrasidlo (U.S. Pat. No. 4,629,563) asymmetric membrane.
Asymmetric or anisotropic membranes are well known in the art. For example, Wrasidlo in U.S. Pat. Nos. 4,629,563 and 4,774,039 and Zepf in U.S. Pat. Nos. 5,188,734 and 5,171,445, the disclosures of which are hereby incorporated by reference, each disclose asymmetric membranes and methods for their production. The Wrasidlo patent disclosed the first truly asymmetric microfiltration membrane. "Asymmetric" as used in the context of the Wrasidlo patent refers to membranes that possess a progressive change in pore size across the cross-section between the microporous skin and substructure. This stands in contrast to reverse osmosis and most ultrafiltration membranes which have abrupt discontinuities between a "nonmicroporous skin" and the membrane substructure, and which are also referred to in the art as asymmetric.
Each of the Wrasidlo and Zepf patents discloses highly asymmetric, integral, microporously skinned membranes, having high flow rates and excellent retention properties. The membranes are generally prepared from a modified "phase inversion" process using a metastable two-phase liquid dispersion of polymer in a solvent/nonsolvent system which is cast and subsequently contacted with a nonsolvent. The Zepf patent discloses an improvement over the Wrasidlo patent.
Phase inversion processes generally proceed through the steps of: (i) casting a solution or a mixture comprising a suitably high molecular weight polymer(s), a solvent(s), and a nonsolvent(s) into a thin film, tube, or hollow fiber, and (ii) precipitating the polymer through one or more of the following mechanisms:
(a) evaporation of the solvent and nonsolvent (dry process); PA1 (b) exposure to a nonsolvent vapor, such as water vapor, which absorbs on the exposed surface (vapor phase-induced precipitation process); PA1 (c) quenching in a nonsolvent liquid, generally water (wet process); or PA1 (d) thermally quenching a hot film so that the solubility of the polymer is suddenly greatly reduced (thermal process).
Schematically, the inversion in phase from a solution to a gel proceeds as follows: ##STR1##
Essentially, SOL 1 is a homogenous solution, SOL 2 is a dispersion, and the Gel is the formed polymer matrix. The event(s) that triggers SOL 2 formation depends on the phase inversion process used. Generally, however, the triggering event(s) revolves around polymer solubility in the SOL. In the wet process, SOL 1 is cast and contacted with a nonsolvent for the polymer which triggers the formation of SOL 2 which then "precipitates" to a Gel. In the vapor phase-induced precipitation process, SOL 1 is cast and exposed to a gaseous atmosphere including a nonsolvent for the polymer which triggers the formation of SOL 2 which then "precipitates" to a Gel. In the thermal process, SOL 1 is cast and the temperature of the cast film is reduced to produce SOL 2 which then "precipitates" to a Gel. In the dry process, SOL 1 is cast and contacted with a gaseous atmosphere, such as air, which allows evaporation of one or more of the solvents which triggers the formation of SOL 2 which then "precipitates" to a Gel.
The nonsolvent in the casting dope is not always completely inert toward the polymer; in fact it usually is not inert, and is often referred to as swelling agent. In the Wrasidlo-type formulations, as discussed later, selection of both the type and the concentration of the nonsolvent is important in that it is the primary factor in determining whether or not the dope will exist in a phase separated condition.
In general, the nonsolvent is the primary pore forming agent, and its concentration in the dope greatly influences the pore size and pore size distribution in the final membrane. The polymer concentration also influences pore size, but not as significantly as does the nonsolvent. It does, however, affect the membrane's strength and porosity. In addition to the major components in the casting solution, or dope, there can be minor ingredients, such as, for example, surfactants or release agents.
Polysulfone is especially amenable to formation of highly asymmetric membranes, particularly in the two-phase Wrasidlo formulations. These are not homogeneous solutions but consist of two separate phases: one is a solvent-rich clear solution of lower molecular weight polymer at low concentrations (e.g., 7%); the other is a polymer-rich, turbid, colloidal solution of higher molecular weight polymer at high concentrations (e.g., 17%). The two phases contain the same three ingredients, that is, polymer, solvent, and nonsolvent, but in very different concentrations and molecular weight distributions. Most importantly, the two phases are insoluble in one another and, if allowed to stand, will separate. The mix therefore must be maintained as a dispersion, with constant agitation until the time that it is cast as a film. Essentially, in Wrasidlo type formulations, the casting dope is provided in a SOL 2 (dispersion) condition. Thus, the dispersion serves as the starting point for gel formation and not as the intermediate step (above), as follows: ##STR2## This process modification was largely responsible for the higher degrees of asymmetry and uniform consistency of the Wrasidlo Membranes as compared to the prior art.
It is the nonsolvent and its concentration in the casting mix that produces phase separation, and not every nonsolvent will do this. The two phases will separate from one another if allowed to stand, but each individual phase by itself is quite stable. If the temperature of the mix is changed, phase transfer occurs. Heating generates more of the clear phase; cooling does the reverse. Concentration changes have the same effect, but there is a critical concentration range, or window, in which the phase separated system can exist, as discussed by Wrasidlo. Wrasidlo defines this region of instability on a phase diagram of thus dispersed polymer/solvent/nonsolvent at constant temperature, lying within the spinodal or between spinodal and binodal curves, wherein there exist two macroscopically separated layers.
Because of the great hydrophobicity of the polymer and because of the thermodynamically unstable condition of the casting mix, wherein there pre-exist two phases, one solvent-rich and the other polymer-rich (a condition that other systems must pass through when undergoing phase inversion), the unstable Wrasidlo mixes precipitate very rapidly when quenched so as to form a microporous skin at the interface and consequently develop into highly asymmetric membranes, a structure shared by the membranes of each of the Wrasidlo and Zepf patents.
The microporous skin is the fine pored side of the membrane that constitutes the air-solution interface or the quench-solution interface during casting. In the Wrasidlo patent, and in this disclosure, it is understood that the term "skin" does not indicate the relatively thick, nearly impervious layer of polymer that is present in some membranes. Herein, the microporous skin is a relatively thin, porous surface that overlies a microporous region of variable thickness. In an asymmetric membrane, the pores of the microporous region gradually increase in size as they lead from the skin to the opposite face of the membrane. The region of gradual pore size increase is sometimes referred to as the asymmetric region, and the opposite, non-skin face of the membrane is often referred to as the coarse pored surface. As a contrast to the coarse pored surface, the skin is also sometimes called the microporous surface.
In some formulations and casting conditions, a "skinning" effect can occur at the opposite surface of the membrane--the surface that is in contact with the casting support and that is not exposed directly to humid air or to the quench bath in the casting process. Where such a layer of "opposite skin" exists, it is usually relatively thin, typically being less than about 10% of the thickness of the membrane. It is also usually highly porous, although the porous network or web may have a more densely packed appearance in cross section than the internal region of the membrane between the two skins, if both are present.
Polymeric membranes can also be cast from homogeneous solutions of polymer. The composition of these formulations lies outside of the spinodal/binodal region of the phase diagram of Wrasidlo. Membranes cast from homogeneous solutions may also be asymmetric, although they are not usually as highly asymmetric as those cast from phase separated formulations.
The Wrasidlo membranes have improved flow rates and permselectivity in relation to prior art membranes. Such improved flow rates and permselectivity arise from the structure of the membranes.
The Zepf patents disclose improved Wrasidlo-type polymer membranes having a substantially greater number of microporous skin pores of more consistent size, and greatly increased flow rates, with reduced flow covariance for any given pore diameter. The improved Zepf membranes are achieved by modifications to the Wrasidlo process, comprising reduced casting and quenching temperatures, and reduced environmental exposure between casting and quenching. Zepf further teaches that reduced casting and quenching temperatures minimize the sensitivity of the membrane formation process to small changes in formulation and process parameters.
The Wrasidlo patents also disclose the preparation of PVDF asymmetric membranes. See U.S. Pat. No. 4,774,039, Example 6, Column 12, lines 20-34. However, the PVDF membranes prepared in accordance with the Wrasidlo patent do not appear to have been microporous membranes.
None of the microporous PVDF membranes of the prior art discussed herein possesses a highly asymmetric structure. Consequently all prior art PVDF membranes are limited in their flow rates as compared to highly asymmetric membranes. Therefore, as will be appreciated by those of ordinary skill in the art, it would be desirable to provide a microporous PVDF membrane having a high degree of asymmetry and high flow rates. Further, it would be beneficial to provide ultrathin microporous PVDF membranes having high flow rates, whether isotropic or asymmetric in structure. In addition, it would be desirable to provide highly asymmetric PVDF membranes that are either hydrophobic or hydrophilic. It would also be beneficial to provide methods to enable the consistent production of each of such membranes.